ELAM Example: Nexus7000 F2


Tue, 09/10/2013 - 07:40
Sep 7th, 2013
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This document provides the steps to perform an ELAM on the Nexus 7000 F2 modules, explains the most relevant outputs, and how to interpret their results.  Please refer to the following document for an overview on ELAM:

Understanding ELAM



In this example a host on Vlan10 ( port Eth6/4 sends an ICMP request to a host also on Vlan10 ( off port Eth6/3.  We will use ELAM to capture this single packet between the hosts.  It's important to remember that ELAM allows us to capture a single frame.

To perform an ELAM on the Nexus7000, you need to first attach to the appropriate module.  This requires the network-admin privilege.

N7K# attach module 6

Attaching to module 6 ...

To exit type 'exit', to abort type '$.'


Determine the ingress FE

We expect the traffic to ingress the switch on port Eth6/4.  Checking the modules in the system we can see that module 6 is an F2 module.  Remember, the Nexus 7000 is fully distributed and the modules, not the supervisor, are responsible for making the forwarding decision for dataplane traffic.

N7K# show module 6

Mod  Ports  Module-Type                         Model              Status

---  -----  ----------------------------------- ------------------ ----------

6    48     1/10 Gbps Ethernet Module          N7K-F248XP-25E     ok

For F2 modules, we want to perform the ELAM on the L2 forwarding engine (FE) with internal codename Clipper.  Note that the L2 FE data bus (DBUS) contains original header information before the L2 and L3 lookup and the result bus (RBUS) contains the results after both L3 and L2 lookups. 

The F2 module has 12 forwarding engines per module.  We need to determine which Clipper ASIC is the FE for port Eth6/4.  We can use the following command to verify:

module-6# show hardware internal dev-port-map


CARD_TYPE:         48 port 10G

>Front Panel ports:48


Device name             Dev role              Abbr num_inst:


> Clipper FWD            DEV_LAYER_2_LOOKUP     L2LKP 12


+----------------+++FRONT PANEL PORT TO ASIC INSTANCE MAP+++------------+




   3       0       0       0       0       0       0      

   4       0       0       0       0       0       0  

From the output above, we can see that Eth6/4 is on Clipper (L2LKP) instance 0. 

module-6# elam asic clipper instance 0

module-6(clipper-elam)# layer2


Configure the Trigger

The Clipper ASIC supports ELAM triggers for multiple frame types. The ELAM trigger must align to the frame type.  If the frame is an IPv4 frame then the trigger must also be IPv4.  An IPv4 frame will not be captured with an "other" trigger.  The same logic applies to IPv6.  You can see the different frame types supported by Clipper below:

module-6(clipper-l2-elam)# trigger dbus ?

  arp    ARP Frame Format

  fc     Fc hdr Frame Format

  ipv4   IPV4 Frame Format

  ipv6   IPV6 Frame Format

  other  L2 hdr Frame Format

  pup    PUP Frame Format

  rarp   Rarp hdr Frame Format

  valid  On valid packet

For NX-OS you can utilize the question mark to help parse out the ELAM trigger.  You'll notice that there are several options available for F2 ELAM.

module-6(clipper-l2-elam)# trigger dbus ipv4 ingress if ?


  destination-ipv4-address         destination ipv4 address

  destination-mac-address          Inner destination mac address

  source-index                     Source index

  source-ipv4-address              source ipv4 address

  source-mac-address               Inner source mac address

  vlan                             Vlan


For this example we want to capture the frame based off source and destination IPv4 address so we will only specify those values.

Clipper requires a trigger to be set for the DBUS and the RBUS.  Different from M-series modules, there is no requirement to specify a packet buffer instance which helps simplify the RBUS trigger.

DBUS Trigger

module-6(clipper-l2-elam)# trigger dbus ipv4 ingress if source-ipv4-address destination-ipv4-address

RBUS Trigger

module-6(clipper-l2-elam)# trigger rbus ingress if trig

Start the Capture

Now that the ingress FE has been selected and we've configured our trigger, we can start the capture

module-6(clipper-l2-elam)# start

We can check the status of the ELAM via the status command.

module-6(clipper-l2-elam)# status

ELAM instance 0: L2 DBUS Configuration: trigger dbus ipv4 ingress if source-ipv4-address destination-ipv4-address

L2 DBUS Armed

ELAM instance 0: L2 RBUS Configuration: trigger rbus ingress if trig

L2 RBUS Armed

Once the frame matching the trigger has been received by the FE we will see the ELAM as triggered:

module-6(clipper-l2-elam)# status

ELAM instance 0: L2 DBUS Configuration: trigger dbus ipv4 ingress if source-ipv4-address destination-ipv4-address

L2 DBUS Triggered

ELAM instance 0: L2 RBUS Configuration: trigger rbus ingress if trig

L2 RBUS Triggered

Interpret the Results

We can display the results via the show dbus and show rbus command. Below is an excerpt of the ELAM data that is most relevant in this example.

(some output omitted)

module-6(clipper-l2-elam)# show dbus


                       L2 DBUS CONTENT - IPV4 PACKET                



vlan                : 0xa             destination-index   : 0x0          

source-index        : 0x3             bundle-port         : 0x0  

sequence-number     : 0x3f            vl                  : 0x0




destination-mac-address 0050.56a1.1aef

source-mac-address: 0050.56a1.1a01

module-6(clipper-l2-elam)#  show rbus


                       L2 RBUS INGRESS CONTENT                             


l2-rbus-trigger     : 0x1             sequence-number     : 0x3f          

di-ltl-index        : 0x2             l3-multicast-di     : 0x0          

source-index        : 0x3             vlan-id             : 0xa

From the DBUS data above we can validate the frame was received on Vlan10 (vlan: 0xa) with a source MAC of 0050.56a1.1a01 and a destination MAC of 0050.56a1.1aef.  We can also see that this is an IPv4 frame sourced from destined to  There are several other fields not included in this output such as TOS value, IP flags, IP length, L2 frame length, etc... that are also often useful to check.

We can also validate what port the frame was received on via the source-index (the source LTL).  For Nexus 7000, we can map an LTL to a port or group of ports via the following command:

N7K# show system internal pixm info ltl 0x3

Type            LTL


PHY_PORT       Eth6/4

The above output shows that source-index of 0x3 maps to port Eth6/4.  This confirms that the frame was received on Eth6/4.

From the RBUS data we can validate that the frame was switched on Vlan10 (vlan-id: 0xa).  We can confirm the egress port from the di-ltl-index (destination LTL):

N7K# show system internal pixm info ltl 0x2

Type            LTL


PHY_PORT       Eth6/3

The above output shows that the di-ltl-index of 0x2 maps to port  Eth6/3.  This confirms that the frame was switched out Eth6/3.

Further Information

Another command to remember is show system internal pixm info ltl-region, which will show how the switch has allocated the pool of LTL's.  This is useful to understand the purpose of an LTL if it does not match to a physical port.  A good example is a drop LTL:

N7K# show system internal pixm info ltl 0x11a0

0x11a0 is not configured

N7K# show system internal pixm info ltl-region

LTL POOL TYPE                          SIZE        RANGE


DCE/FC Pool                            1024       0x0000 to 0x03ff

SUP Inband LTL                           32       0x0400 to 0x041f

MD Flood LTL                              1       0x0420

Central R/W                               1       0x0421

UCAST Pool                             1536       0x0422 to 0x0a21

PC Pool                                1720       0x0a22 to 0x10d9

LC CPU Pool                              32       0x1152 to 0x1171

EARL Pool                                72       0x10da to 0x1121

SPAN Pool                                48       0x1122 to 0x1151

UCAST VDC Use Pool                       16       0x1172 to 0x1181

UCAST Generic Pool                       30       0x1182 to 0x119f

LISP Pool                                 4       0x1198 to 0x119b

Invalid SI                                1       0x119c to 0x119c

ESPAN SI                                  1       0x119d to 0x119d

Recirc SI                                 1       0x119e to 0x119e

Drop DI                                   2       0x119f to 0x11a0

UCAST (L3_SVI_SI) Region                 31       0x11a1 to 0x11bf

UCAST (Fex/GPC/SVI-ES)       3648       0x11c0 to 0x1fff

UCAST Reserved for Future Use Region   2048       0x2000 to 0x27ff

======================> UCAST MCAST BOUNDARY <======================

VDC OMF Pool                             32       0x2800 to 0x281f



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